In addition to air pollution derived from auto engine exhaust gases and crank case vapors, it is known that vapors from gasoline storage tanks and carburetors contribute in some degree to atmospheric contamination. With respect to the carburetor, there is a desire to obtain proper gasoline flow from the float bowl to a carburetor mixing chamber and this requires suitable vent means from the float bowl to maintain atmospheric pressure therein. In one approach, prevention of excessive vapor pressure in the float bowl is provided by a vent line from the bowl directly to the atmosphere (such an arrangement is referred to as an external vent system). Another approach is to use a direct line or passageway from the inside of the float bowl to the air intake portion of the carburetor, upstream of the venturi mixing section, such as at the air cleaner (this is referred to as an internal system).
Fuel vapor is released to the atmosphere particularly during an engine condition known as "hot soak"; the evaporation losses of gasoline from the carburetor float bowl after the engine is stopped and during hot weather periods, can be quite appreciable.
The principal approach of the prior art has been to eliminate such vapor releases by providing a hydrocarbon absorption bed to pick up gasoline vapors from the carburetor or storage system and retain such vapors in the bed during idle or slow engine speed conditions, as well as during hot soak conditions. The vapors are desorbed from the device during higher speed engine operation. The transition from absorption to desorption is brought about by suitable valve means incorporated into the system.
The interposition of absorption material, in a line communicating with such vapors, does not always insure that optimum or effective storage of the vapors will take place during absorbing conditions. The introduction of vapors to one side of an absorption bed will not necessarily permit the vapors to seek penetration and effectively move throughout the entire absorption bed. One reason for this is that there must be access to an air vent for purging. Typically, this vent is open continuously to avoid the cost of extra equipment. Such vent must be located on the opposite side of the bed to eliminate short circuiting of the incoming vapors directly to the vent. As a result, the column of carbon between the inlet and vent becomes the determining factor in the collecting capacity of the bed. Thus, an increase in other dimensions of the bed has little effect on its absorbing capability. Efficiency of such absorption system is decreased accordingly.